Businesses and consumers are demanding high speed data (HSD), voice over IP (VoIP) and video services (CATV, IPTV), therefore access communications networks need to be designed to keep up with this demand. Telephone companies and Multisystem Operators (MSOs) satisfy this demand by bringing optical fiber deeper into the network. This is typically done by deploying a passive optical network (PON) such as a fiber-to-the-premises (FTTP), fiber-to-the-curb (FTTC), fiber-to-the-home (FTTH) or Radio Frequency over Glass (RFoG) network to deliver these services to the subscriber.
There are two general system architectures deployed in PON networks, time-division multiple access (TDMA) and frequency-division multiple access (FDMA). The TDMA method is used in EPON or GPON networks where the customer premises equipment (CPE) or optical network unit (ONU) is assigned a time slot and transmits only within its allotted time. The FDMA method is typically found in RFoG networks. FIG. 1 shows a typical example of an RFoG network. In the downstream direction, video, and data modulated as AM-VSB and QAM RF Carriers is optically modulated by an optical transmitter, amplified in the optical domain by an erbium doped fiber amplifier (EDFA) and transported to the CPE at the subscriber site over fiber. The CPE converts the optical signal into RF and the RF signals are delivered to the set top box and cable modem over coaxial cable. In the upstream direction, the signals carrying set top box data and upstream data from the cable modem are converted from an RF signal to an optical signal and transmitted to an optical receiver where they are converted back to RF and distributed to the upstream ports of the cable modem termination system (CMTS) and the set top box controller.
The CMTS in the multiple system operator (MSO) headend, or hub and the cable modems at the subscriber sites form a point to multi-point communication network. In the downstream direction, the RF carriers from the CMTS are continuously on. In the upstream direction, since several cable modems communicate with the single CMTS upstream receiver, both Time Division Multiple Access (TDMA) and Frequency Division Multiple access (FDMA) are used. Multiple RF frequencies can be assigned to groups of cables modems, and within a group of cable modems that use a specific RF frequency, TDMA is used to avoid data collisions. However any two (or more) cable modems that are operating at different RF frequencies can transmit at the same time. When this happens, the lasers of the CPEs that they are connected to are also activated and there is a non-zero statistical probability that the laser wavelengths of those CPEs can overlap. It is critical to avoid this event in any RFoG system because when two (or more) optical signals of the same wavelength or with wavelengths that are close together are incident on an optical receiver, an optical effect known as Optical Beat Interference (OBI) can severely degrade of the signal-to-noise ratio (SNR) over a large RF bandwidth (FIG. 2) resulting in a loss of data.
In RFoG networks, CPE lasers operate in a burst mode configuration. When the RF level of the upstream signal crosses a threshold level, the laser is turned on. When it drops below a certain threshold level, the laser is turned off. This burst mode operation reduces the probability of OBI, but does not eliminate it, as discussed above. In HFC networks, the upstream lasers are on continuously and therefore the probability of OBI is significantly higher.